What does Cassius reveal to Casca in Julius Caesar?

Cassius tells Casca that they have many like-minded Romans ready to act against Caesar.

Cassius seems to be going around recruiting assassins in this first act.  He convinces Brutus to work with him against Caesar.  He also tells Casca that he has been recruiting other Romans for their efforts.

Now know you, Casca, I have moved already
Some certain of the noblest-minded Romans
To undergo with me an enterprise
Of honourable-dangerous consequence … (Act 1, Scene 3)

Cassius wants to have as many people involved as possible so that they all are in it together.  He tells Cassius that he has been convincing others, so he must have been at this for a while.  Cassius and the others want to make sure Brutus is on board.  They need his name and reputation to lend legitimacy to the cause.

Casca, for his part, is highly opposed to Caesar.  He says that he would rather die than be Caesar's slave.  Casca and the others think that Caesar is making himself out to be king, and will gather more and more power until he is unstoppable.  Casca is the first to stab Caesar, and his wound is the fatal one.

Cassius and Casca are sneaky in convincing Brutus.  They leave letters for Brutus to find that will make him think that they are from various people, asking him to act against Caesar.

CASSIUS

Be you content: good Cinna, take this paper,
And look you lay it in the praetor's chair,
Where Brutus may but find it … (Act 1, Scene 3)

Brutus finds the letter, and in his speech where he says “it must be by his death,” he seems to talk himself into believing that Caesar is too dangerous to be left alive.  All of Rome is for Brutus acting against Caesar, he thinks.  Brutus believes that killing Caesar is what is best for Rome.